This can be very useful for setting up **resources** that you need to use for the whole app, and that are **shared** among requests, and/or that you need to **clean up** afterwards. For example, a database connection pool, or loading a shared machine learning model.

## Use Case { #use-case }

Let's start with an example **use case** and then see how to solve it with this.

OAuth2 was designed so that the backend or API could be independent of the server that authenticates the user.

But in this case, the same **FastAPI** application will handle the API and the authentication.

So, let's review it from that simplified point of view:

* The user types the `username` and `password` in the frontend, and hits `Enter`.

    @Required
    public String target;

    /**
     * The type of script that was executed for this job.
     * This is a required field indicating the script engine or type used.
     */
    @Required
    public String scriptType;

    /**
     * The script data or code that was executed.
     * This field contains the actual script content that was run.
     */
    public String scriptData;

    /**

/**
 * Describes a problem that was encountered during model building. A problem can either be an exception that was thrown
 * or a simple string message. In addition, a problem carries a hint about its source, e.g. the POM file that exhibits
 * the problem.
 *
 */
public interface ModelProblem extends BuilderProblem {

    /**
     * Enumeration of model versions that can be validated.

  }

  /*
   * Our implementation works fine with a null `dir`. However, there's nothing in the documentation
   * of the supertype that suggests that implementations are expected to tolerate null. That said, I
   * see calls in Google code that pass a null `dir` to a FilenameFilter.... So let's declare the
   * parameter as non-nullable (since passing null to a FilenameFilter is unsafe in general), but if

    /**
     * Test getTargetName() method with a string input.
     * Verifies that the original string is returned after encoding and decoding.
     */
    @Test
    void testGetTargetNameFromStringConstructor() {
        String targetName = "MyService/Host.Domain.com";
        AvTargetName avTargetName = new AvTargetName(targetName);

        // Verify that getTargetName returns the original string

### Required, can be `None` { #required-can-be-none }

You can declare that a parameter can accept `None`, but that it's still required. This would force clients to send a value, even if the value is `None`.

To do that, you can declare that `None` is a valid type but simply do not declare a default value:

{* ../../docs_src/query_params_str_validations/tutorial006c_an_py310.py hl[9] *}

    assume().that(graphIsMutable()).isTrue();
    assume().that(graph.allowsSelfLoops()).isTrue();

    graphAsMutableGraph.putEdge(N1, N1);
    assertThat(graphAsMutableGraph.putEdge(N1, N1)).isFalse();
  }

  @Test
  public void removeEdge_antiparallelEdges() {
    assume().that(graphIsMutable()).isTrue();

    putEdge(N1, N2);
    putEdge(N2, N1);

    protected String[] cacheResponseFields;

    /** Array of fields that can be highlighted in search results */
    protected String[] highlightedFields;

    /** Array of fields that can be searched against */
    protected String[] searchFields;

    /** Array of fields that can be used for faceted search */
    protected String[] facetFields;

    /** Array of fields that can be used for sorting search results */

   * @param event the event that could not be delivered.
   */
  public DeadEvent(Object source, Object event) {
    this.source = checkNotNull(source);
    this.event = checkNotNull(event);
  }

  /**
   * Returns the object that originated this event (not the object that originated the
   * wrapped event). This is generally an {@link EventBus}.
   *